1. Field of the Invention
The present invention relates to a measuring device used for an alignment optical system and a position measurement optical system for various patterns formed on a wafer and an adjustment method thereof.
2. Related Background Art
As disclosed in Japanese Patent Application Laid-Open No. 2001-317913, a position measurement optical system for various patterns formed on a wafer is conventionally composed of an illumination optical system, an imaging optical system and an image processing means as shown in FIG. 4. Incidentally, FIG. 4, which is a prior art, is a schematic view of a position measurement optical system for various patterns.
In the illumination optical system, a filament image of a light source 1 such as a halogen lamp is projected on an incident plane of an optical fiber 4 by a first and a second light source relay lenses 2 and 3. The illumination light is projected on an illumination aperture stop 7 by a first and a second illumination relay lenses 5 and 6 through the optical fiber 4.
Then, the illumination light passes through a third illumination relay lens 8, a field stop 9, and a fourth illumination relay lens 10, and is reflected by an epi-illumination prism 11 by an amount of 90 degrees to illuminate a wafer 14 through an imaging aperture stop 12 and a first objective lens 13. The wafer 14 on which a pattern to be measured is formed is placed on a wafer holder 15 that is disposed on a moving stage 16.
In the imaging optical system, the light reflected from the pattern to be measured formed on the wafer 14 passes again through the first objective lens 13, and forms an image on an imaging surface of a CCD camera 18 after passing through the imaging aperture stop 12, the epi-illumination prism 11, and a second objective lens 17.
In the image processing means, after the image of the pattern to be measured projected on the imaging surface of the CCD camera 18 is converted into a digital signal, the image is sent to an image processor 19 so that the position measurement with respect to the pattern to be measured is carried out.
Incidentally, in a measuring device using the above-described optical microscope, optical errors such as a telecentric error of an illumination optical system, that of an imaging optical system, and coma significantly effect measurement accuracy. Accordingly, various measures to improve accuracy have been devised.
For example, there is a method to use an adjustment pattern having a difference of an eighth of the illumination wavelength, which is sensitive to detect an error in the optical system. By using the reference pattern, the error in the optical system can be presumed from variation in symmetry of the optical image with respect to an offset amount of focusing, so that an adjustment mechanism portions such as the illumination aperture stop 7, the imaging aperture stop 12, and the second objective lens 17 can be adjusted to the optimum position.
However, an illumination light irregularity of the illumination optical system has not been regarded as a problem since the effect thereof to the accuracy is small.
In order to reduce the illumination light irregularity, a sufficient effect has been obtained by inserting an optical diffuser such as a lemon skin filter into the illumination optical system.
As shown in FIG. 4, a lemon skin filter 20 as an optical diffuser is disposed between the exit surface of the optical fiber 4 and the first illumination relay lens 6.
However, with increasing degree of integration of a semiconductor device and the amount of memory, accuracy to be required to a measuring device is getting dramatically severe, so that the illumination light irregularity is no longer able to be ignored.
Although the optical diffuser such as a lemon skin filter is inserted in the vicinity of the exit surface of the optical fiber, it is impossible to completely remove the light amount irregularity on the exit surface of the optical fiber.
Generally, an optical fiber bundle is constructed such that a plurality of fibers, each composed of core and cladding, are twisted each other. Since there are small gaps between fibers, the gaps become a cause of the light amount irregularity. Moreover, it is unavoidable that there includes a several percentage of snapped fibers upon fabrication. Furthermore, the light amount irregularity and the position of the filament image of the halogen lamp projected on the incident surface of the optical fiber can also be a cause of the light amount irregularity on the exit surface of the optical fiber.
The diameter of the core on the exit surface of the optical fiber is sufficiently large with respect to the diameter of the illumination pupil plane, so that it has been constructed such that strict positional alignment is not necessary. Therefore, an adjustment mechanism has not been attached to the device so far.
However, as increasing degree of integration of a semiconductor device as well as the memory capacity, there is a serious possibility that the measuring device causes a large illumination light irregularity caused by a light amount irregularity on the exit surface of the optical fiber.